The 65 groups, which hail from 31 countries, aren't the first to point out Internet.org's potential problems. As it's currently set up, Internet.org allows people to access websites from a smartphone without a data plan, but very few sites (including Facebook) are actually accessible. So far, it is operating in India, Kenya, Zambia, Tanzania, Ghana, and Colombia, with plans to expand to 100 countries by the end of the year.

You don't have to be all that paranoid to see it more as a play to indoctrinate the developing world into a Facebook-controlled internet, which is exactly what's happened in places like Indonesia, the Philippines, and Thailand, where many people believe that Facebook is the internet.

"The platform creates a two-tier internet that exacerbates, rather than bridges, the digital divide"

"We are especially concerned that access for impoverished people is construed as justification for violations of net neutrality," the groups wrote in a Facebook note directed at Zuckerberg. "It is our belief that Facebook is improperly defining net neutrality in public statements and building a walled garden in which the world's poorest people will only be able to access a limited set of insecure websites and services."

Signers from the United States include Access, Fight for the Future (which was heavily involved in the net neutrality fight), and Media Alliance. Worldwide, groups like the Zimbabwe Human Rights NGO Forum, India's IT for Change, the Netherlands' Bits of Freedom and Baaroo Foundation, and Pakistan's Digital Rights Foundation signed the letter, among dozens of others.

"We are deeply concerned that Internet.org has been misleadingly marketed as providing access to the full Internet, when in fact it only provides access to a limited number of Internet-connected services that are approved by Facebook and local ISPs," they wrote. "In its present conception, Internet.org thereby violates the principles of net neutrality, threatening freedom of expression, equality of opportunity, security, privacy and innovation."

The most obvious problem with Internet.org, they say, is that websites must be specifically approved by Facebook and the country they operate in to achieve what's known as a "zero rating," meaning that accessing the site through Internet.org won't count against a customer's cell phone data plan.

"This practice is inherently discriminatory," the groups wrote, adding that it's been specifically outlawed in Canada, the Netherlands, Slovenia, and Chile.

"The need to extend the internet to the billions who don't have it is real," Josh Levy, advocacy director at Access, said in an email. "But we fundamentally disagree with Facebook about the direction Internet.org is taking. The platform creates a two-tier internet that exacerbates, rather than bridges, the digital divide."

Zuckerberg said that some access is better than no access, and added that it's "not sustainable to offer the whole internet for free."

"Net neutrality should not prevent access," he said in a May 4 video. "It's not an equal internet if the majority of people can't participate."

Update: A Facebook spokesperson sent the following message:

"We and our critics share a common vision of helping more people gain access to the broadest possible range of experiences and services on the internet. We are convinced that as more and more people gain access to the internet, they will see the benefits and want to use even more services. We believe this so strongly that we have worked with operators to offer basic services to people at no charge, convinced that new users will quickly want to move beyond basic services and pay for more diverse, valuable services."